Maintaining Graphics Line on Background image in c# panel

Question

I am drawing lines on a background image in a c# panel. The panel is anchored to the form so as the form resizes the panel resizes. The background image is set to be stretched so all you see as you resize the form is the background image.

My initial problem: The lines drawn on the panel (via the OnPaint event) stay where they were originally drawn as the image resizes.

My current solution: Record the location of the line and redraw it on a new bitmap by scaling the X and Y coordinates (works fine).

My new problem: As you continually resize the window and draw lines you can't calculate the scaling factor from any point in time and apply it to all lines since the lines were originall drawn in different size images.

The two options I think I have: After I redraw the line go through my array of lines and update the coordinate information so it now matches the current scale. Or In addition to storing the coordinate information of the line also store the size information of the panel at the time it was drawn so I can always calculate the scale for each line based on when it was drawn and the new panel size.

What I'm hoping for: If you have thoughts on either of the two approaches that would be greatly appreciated....Even better would be to point me in the direction of a far better method to do this (I am fairly new to graphics processing in c#).

Answer 1

Can't write a comment, much as I want to. You do have a few options:

• Draw your lines directly on the original Bitmap. This might not be an option for you, depending on the task.

• Do it as you're doing now, keeping track of the lines' coordinates, updating them on resize, and redrawing them on Paint - if you use this, you'll be able to move and delete them, too,

• Or do it by introducing a "scale factor" (float) which you update on every resize, and in your Paint event handler you draw everything using that scale factor. As you create a line, you calculate its coordinates using the scale factor BACK TO an unified coordinate system (scale factor 1) and then you don't have to modify your coordinates at all. This might be easy to debug due to that unified coordinate system. This is what I'd recommend, but it again depends on your task.

• Draw to a full transparent Bitmap of the same size as your original image, use a scale factor like in the previous option. On creating a line, calculate its coordinates in the unified coordinate system, draw it on the Bitmap, and then on every Paint, draw the entire Bitmap over your original one. This, again, might not be an option if you need to delete or move your lines, or if you're tight on memory, or you don't want your lines to be blurred when you scale up, but somehow many ppl like this because it's like a "layer in Photoshop". :)